1. Field of the Invention
The present invention relates to an improvement of an auto focus circuit adopted in a video camera.
Particularly, the present invention relates to an improvement of an auto focus circuit of a system in which an input video signal is evaluated to control the movement of a focusing ring by a closed loop.
2. Description of the Prior Art
In the prior art concerning an auto focus circuit of a video camera, two methods are known: a method in which the movement of a focusing ring is controlled by an open loop and a method in which the movement of a focusing ring is controlled by a closed loop by evaluating a video signal. The former method has disadvantages that erroneous operation is liable to occur due to changes in the environmental conditions such as application of an external light and the range of measurement cannot be changed according to the change in a zoom state. As a result, there is an increasing tendency to use the latter method recently.
In the latter closed-loop control method, generally, a high frequency component of a luminance signal corresponding to the central portion of a screen region recorded is sampled and an absolute value of the high frequency component is obtained so that focusing is controlled according to the magnitude of the absolute value. A concrete example of such technology is shown for example on pages 75 to 80 of the journal "Television Technics & Electronics" issued by Electronics Technical Publishing Company in February 1983.
FIG. 1 is a block diagram showing an example of such a well-known auto focus circuit for a video camera as described above.
Referring to FIG. 1, a luminance signal Y contained in a video signal obtained by video recording is applied to a gate circuit 1. The gate circuit 1 selects a luminance signal Y corresponding to the sampling area in the center of the screen region. For this purpose, a gate control circuit 2 controls an open period of the gate circuit 1 based on the output of a synchronizing separation circuit 3. The output of the gate circuit 1 is applied to a high-pass filter 4 for cutting off the signal lower than 100 kHz. The output of the filter 4 attains the maximum level in a state in which the video camera is focused. This is because as the outline of an object to be recorded comes into focus to become clearer, a more marked change is produced in the rise or the fall of an input luminance signal Y in the outline portion and the high frequency component becomes larger. The output of the high-pass filter 4 is supplied to an absolute value calculating circuit 5 and then it is integrated for each field in an average value calculating circuit 6. The integrated average value output is converted to a digital signal with a cycle of one field in an analog-to-digital (A-D) converting circuit 7. The converted output is written in a first memory circuit 8 and then written in a second memory circuit 9 after one field. The writing in the first memory circuit 8 and the second memory circuit is controlled by a vertical synchronizing signal V from the synchronizing separation circuit 3.
The output of the first memory circuit 8 and the output of the second memory circuit 9 are compared in a comparing circuit 10 and the output of comparison is supplied to a focusing motor control circuit 11. The focusing motor control circuit 11 rotates in an initialized state, a focusing motor (not shown) in a predetermined direction and the rotation in this direction is maintained as far as the output of the first memory circuit 8 is larger than the output of the second memory circuit 9. On the contrary, if the output of the first memory circuit 8 is smaller than the output of the second memory circuit 9, the focusing motor is controlled to rotate in the opposite direction. Thus, the focusing motor is controlled so that the average level of the high frequency component contained in the luminance signal is always maximum.
However, if the absolute value output is averaged in the form of an analog signal not converted, the average level is considerably changed dependent on the scene to be recorded. More specifically stated, the average output for recording a scene where there is a considerable change of the luminance in the horizontal direction is ten times as large as the average output for recording a scene where there is little change of the luminance in the horizontal direction. As a result, it is difficult to determine an A-D conversion range for the average output the value of which increases or decreases considerably. Accordingly, if the A-D conversion range is adapted for a high average output, the resolution for a low average output is decreased and the function of response for control is deteriorated. On the contrary, if the A-D conversion range is adapted for a low average level, a saturation of the conversion range with respect to a high average output becomes a problem.
For this reason, in a conventional circuit, the A-D conversion range is adapted for a low average value output and an average value output level exceeding the A-D conversion range is detected by a saturation level detecting circuit 12 so that the output of the gate control circuit 2 is controlled. By this control, scanning lines in the sampling area are decreased by every other line, every two lines, every three lines, etc. so that the average output is not saturated.
Accordingly, if there is a considerable change in the luminance in the horizontal direction of an object to be recorded, sampling intervals are made less frequent. In the case of an object having little correlation in the vertical direction, erroneous operation might occur in focusing.